1. Field of the Invention
This invention relates generally to a grid for use with X-rays. More particularly, this invention relates to a grid for reducing X-ray scatter. Still more particularly, this invention relates to a grid for simultaneously reducing X-ray scatter in more than one direction.
2. Description of the Related Art
X-rays are commonly used to produce images in a variety of settings, including medical diagnosis. X-rays are electromagnetic radiation of extremely short wavelengths and high energy. This is true even in the range of energies used for medical diagnosis. When an X-ray encounters an atom of matter, it may be absorbed or deflected. The deflected X-rays make up what is known as scatter. Scatter serves no useful purpose in making the final image, and distracts from the clarity of the image. Taking the field of medical diagnosis as an example, the image will ideally be made by only those X-rays that have passed directly through the patient without colliding with atoms along the path. At any given point of the image, the quantity of X-rays at that point indicates the degree of absorption of the primary beam in the patient on the line from the X-ray source to the X-ray receptor (e.g., the film). The scattered X-rays arrive at the X-ray film from various angles and places in the body not related to the path from the source to the receptor. The unwanted scattered X-rays cause the image to appear clouded. This reduces the image contrast, and obscures small variations that exist within the body being imaged.
One way to reduce X-ray scatter is through the use of a scatter reduction grid. Scatter reduction grids are made up of spaced-apart X-ray absorbing strips. FIG. 1 shows a known scatter reduction grid 100. The grid 100 is placed between an object to be imaged, such as a patient 102, and a receptor 104. Ideally, the grid 100 will allow unimpeded passage of an X-ray beam 108 that has come straight from the X-ray source 106 through the patient 102 and will absorb all of the X-ray beams 110 that were scattered by passage through the patient 102. However, as seen in FIG. 2, if the strips are made tall in relation to their spacing (a xe2x80x9chigh ratioxe2x80x9d grid 200), they will stop most or all of the scattered rays 110, but they will also stop many of the desired primary rays 108. If the strips are short with respect to the spacing (a xe2x80x9clow ratioxe2x80x9d grid 300), they will allow the primary rays 108 to pass through easily, but some of the scattered X-rays 110 will also pass through.
A partial solution to this problem is the use of a focused grid 400. As seen in FIG. 3, the strips of a focused grid 400 are parallel to each other in their longitudinal direction, but lean toward each other in the direction of X-ray propagation. This allows more primary rays 108 to pass through the grid. However, the focused grid 400 performs well at only one particular distance from the X-ray source 106, since if not at its proper location, the grid 400 will trap many of the primary X-rays 108 as well as the scattered X-rays 110.
The grids described thus far are linear grids. That is, they only reduce or remove the scatter in one dimension. If the lines of the grid are oriented in a north-south direction, any scattered rays that come off in a north-south direction will not be removed. The grid will absorb only those rays scattered in an east-west direction. The typical solution is to orient two linear grids orthogonally to each other to create a cross-hatched grid. This process doubles the grid absorption, but significantly improves the image contrast by reducing the amount of scatter that reaches the detector 104. The main drawback to this approach is the removal of too many primary X-rays, requiring a higher radiation dose to the patient.
Thus, what is needed is an improved X-ray scatter reduction grid that allows for reduction of scatter in more than one direction and that removes as few primary X-rays as possible.
The present invention provides an improved scatter reduction grid having a first layer including a plurality of cells. The cells have a perimeter formed of an X-ray absorbing material. The shape of the perimeters can vary, but a polygonal shape is preferred. Preferably, the shape is a triangle, a trapezoid, a rhombus, a pentagon, a hexagon, a heptagon, or an octagon.
Subsequent layers of cells can be also included. The cells of the successive subsequent layers are larger than and offset from the prior layer cells. The increase in size of the cells in each layer depends on a number of factors, including the spacing between the layers and the distance from the radiation source. The increased size of the cells allows a primary ray passing through the center of a cell to also pass through the center of cells in subsequent layers. This allows for a maximum of primary rays to pass through the grid and allows greatly improved scatter absorption.